ABSTRACT The gut is a major site for early HIV-1 infection and CD4+ T cell depletion, and a critical compartment for the antiviral action of the type I interferons (IFN) that include the 12 IFN? subtypes and IFN?. However, the initial IFN response may be suboptimal, as transmitted/founder (TF) HIV-1 strains still manage to break through. Using next-generation sequencing, we reported that the IFN? subtypes expressed by plasmacytoid dendritic cells (pDCs) following HIV-1 exposure ex vivo had relatively weak antiviral activity. These weakly antiviral IFN? subtypes include IFN?2, the only IFN? subtype approved for clinical use, and IFN?1, a potential antagonist of type I IFN signaling. To date, in-depth studies on the regulation and biological properties of the IFN? subtypes and IFN? in primary pDCs and gut cells has not yet been undertaken. Paradoxically, the type I IFNs were also linked to chronic immune activation, a strong predictor of HIV-1 disease progression. The phenotype is likely due to the immunomodulatory properties of the type I IFNs, but the exact mechanisms remain unclear. Of note, gut barrier dysfunction occurs early in HIV-1 infection, leading to the translocation of microbes into the lamina propria, resulting in immune activation. We reported that gram-negative commensal bacteria enriched in the gut mucosa of HIV-1-infected individuals enhanced HIV-1 replication and CD4+ T cell death ex vivo in the gut Lamina Propria Aggregate Culture (LPAC) model. Here, we hypothesize that the transition from a protective to a pathogenic role for type I IFNs may be driven by translocating enteric microbes. Microbial exposure may raise the threshold for the antiviral effects of type I IFNs to manifest and `license' immunomodulatory ISGs to promote myeloid (mDC) activation/trans-infection and CD4+ T cell infection/ apoptosis. These microbe-driven pathogenic effects of type I IFNs may be sustained during chronic infection. Interestingly, we observed that type I IFN responses during chronic HIV-1 infection are compartmentalized in vivo, with differentially enhanced IFN? versus IFN? in the blood versus the gut, respectively. To date, the cellular sources and mechanisms driving the elevated type I IFN signature in the gut remains unknown. We thus propose to investigate the role of type I IFNs in gut HIV-1 infection during the acute stage, at the onset of microbial translocation, and during the chronic stage. In Aim 1, we will evaluate the regulation, anti-HIV-1 activity and functional properties of the IFN? subtypes and IFN?. In Aim 2, we will determine how type I IFNs modulate mDC activation and T cell function/survival in the context of HIV-1-associated gut dysbiosis and microbial translocation. In Aim 3, we will determine the source and triggers of abnormal type I IFN signature during chronic infection using gut tissues from uninfected, untreated HIV-1-infected and HIV-1-suppressed individuals. The results should provide urgently needed insights on how type I IFNs impact HIV-1 pathogenesis that may inform strategies to either harness these antiviral cytokines for curative strategies or block their immune effects to reduce chronic inflammation.